Mercaptans are undesirable because of their unpleasant odor and corrosivity and also because they degrade the stability of end-product fuels. The liquid disulfides created by conversion of the mercaptans do not have these undesirable characteristics and can be retained in the Merox treated fuels or removed and used elsewhere in the petroleum refinery. The Merox process is generally more economical than a catalytic hydrodesulfurization process and achieves much the same result. Economic and practical drawbacks associated with hydrodesulfurization processes include additional dedicated facilities to which the disulfide compounds must be transferred, use of expensive and sensitive catalysts and the treatment and disposal of the by-product sulfur-containing compounds.
Processes in oil refineries and natural gas processing plants that remove mercaptans and/or hydrogen sulfide (H2S) are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide. The liquid hydrocarbon disulfides can remain in the sweetened end products; or they can be used as part of the petroleum refinery or natural gas processing plant fuel; or they may be subjected to further downstream processing.
One proprietary catalytic mercaptan oxidation process widely used in petroleum refineries and natural gas processing plants to remove mercaptans contained in end-products such as LPG, propane, butanes, light naphthas, kerosene and jet fuel by converting them into liquid hydrocarbon disulfides is known as the Merox process. It is an integrated process comprising the mercaptan extraction step in which mercaptans react with an aqueous caustic solution in the presence of a catalyst, to form sodium alkylthiolate, which is then oxidized in a wet air oxidation step to produce disulfides and a regenerated caustic solution which is recycled back to the extraction step. The Merox process requires an alkaline environment which, in some versions of the process, is provided by an aqueous solution of sodium hydroxide (NaOH), a strong base, commonly referred to as caustic. In other versions of the process, the alkalinity is provided by ammonia, which is a relatively weaker base than sodium hydroxide and must be handled with special care due to its irritant and toxicity properties.
The stepwise reaction schemes for the Merox process beginning with the treatment of the mercaptan is as follows:2RSH+2NaOH→2NaSR+2H2O  (1)
In the above reaction, RSH is a mercaptan and R is an organic group such as a methyl, ethyl, propyl or other hydrocarbon group. For example, the ethyl mercaptan (ethanethiol) has the formula C2H5SH.
The catalyst used in some versions of the Merox process is a water-soluble liquid and in other versions, the catalyst is impregnated onto charcoal granules.
The second step is referred to as regeneration and it involves heating and oxidizing the caustic solution leaving the extractor. The oxidation results in converting the extracted mercaptans to organic disulfides (RSSR). These disulfides are water-insoluble liquids that are separated and decanted from the aqueous caustic solution. The regeneration reaction scheme is as follows:4NaSR+O2+2H2O→2RSSR+4NaOH  (2)
On a global basis, Merox mercaptan oxidation units are commonly found in refineries and the disulfides generated are blended with the fuel oil and are typically burned as fuel to produce stream or provide other utilities. This use can raise environmental concerns where the combustion gases with sulfur-containing constituents are emitted in the refinery. In some cases, the disulfides are added to an automotive fuel, or retained as part of the fuel blend; however with increasingly stringent fuel sulfur specifications, it is foreseeable that this use may be eliminated entirely.
The Claus process is a well-established commercial process for recovering elemental sulfur from gaseous hydrogen sulfide found in oil refineries, natural gas processing plants and other industrial facilities. The Claus process includes a thermal and a catalytic step. In the controlled thermal step, one third of the H2S is oxidized to SO2 in a furnace operating at a temperature of about 1000° C. This ensures a stoichiometric reaction for the subsequent catalytic step in which a 2:1 mix of H2S and SO2 passes through a fixed bed of activated alumina or titania-based catalysts maintained at a temperature in the range of from 200°-350° C. to produce elemental sulfur and water.2H2S+SO2→3S+2H2O  (3)
The problem addressed by the present invention is the need for an economical and effective method for the recovery of a clean, sulfur-free hydrocarbon fuel from liquid disulfides, and particularly the hydrocarbon disulfides produced in the caustic processing of mercaptan-containing hydrocarbon product streams, and specifically the Merox process.